The present invention is directed towards packaging for semiconductor devices and more specifically towards a demountable flip chip package which can be easily reworked.
Multi-chip modules which include a number of integrated circuit die in a single package play an increasingly important role in the electronics industry. An important concern in the fabrication of multi-chip modules is the method of electrically connecting the integrated circuit die to the module. Tape Automated Bonding (TAB) and Flip Chip packages are two popular packaging solutions for packaging high lead count VLSI die requiring high thermal dissipation and good electrical performance.
Tape automated bonding involves the formation of a spider-like metal pattern of conductive fingers which radiate outwardly from the integrated circuit die for attachment to contact sites of a substrate. Although TAB interconnection provides a viable means of interconnection for high pin count integrated circuits, it is associated with a number of disadvantages. First, TAB interconnection requires incorporation of an additional element (the tape) to act as an intermediary between the die and substrate thus adding to the signal path. Further, TAB interconnection requires a complicated assembly procedure involving the steps of inner and outer lead bonding. Finally, the high pin count extendability of TAB is limited by its requirement of a peripheral bond pad configuration (as opposed to an area array configuration) on the integrated circuit die.
The flip chip package has an area array configuration increasing the number of interconnections available compared to the standard TAB package. In a flip chip package, the integrated circuit die is fixed face down onto the substrate and solder bumps are formed at the input/output pads of the die. After solder bump formation, the temperature is increased to cause the solder bumps to reflow for direct bonding of the input/output pads of the die to contact sites on the substrate. Although flip chip interconnect schemes eliminate the limitation of peripheral bond pad arrangement associated with TAB bonding, flip chip interconnection has the disadvantage that test and burn-in of the integrated circuit die cannot be performed before board level assembly. Further, flip chip interconnection includes a fairly involved chip-to-substrate assembly procedure which requires the added investment and associated cost of solder bump technology.
In both conventional TAB and flip chip interconnections, the input/output pads or leads are soldered to contact sites on the substrate making it difficult to repair or replace defective die. Conventional TAB technology is based on soldering the tape outer leads to the matching pattern on the substrate. Conventional flip chip technology requires soldering the solder bumps formed on the input/output pads of the integrated circuit die onto matching pads on the substrate. The soldering operation makes rework of the integrated circuit die extremely difficult under production conditions. Because of the difficulty of removing and resoldering the integrated circuit die, the entire multi-chip module may have to be discarded when there is a single defective die. This problem is exacerbated as the number of die on a multi-chip module increases.
One solution to the problem of reworkability is to provide a pressure connection scheme to establish electrical connection of the pads on the die with the matching connection points on the substrate. FIG. 1 illustrates a pressure connection scheme for an electrical assembly. The electrical assembly 100 is comprised of a die pressure plate 102, integrated circuit die 104, a substrate 106, a load distributing member 108 (typically an elastomer), a backing plate 110, and an external load 112. One of many possible aligning schemes (not shown) may be used to align the contact pads on the die with the matching pads on the substrate. In order to establish the electrical connection, the die 104 is aligned to the substrate 106 and the die pressure plate 102 is pressed towards the backing plate 110 by means of an external load 112.
While the above described scheme is fairly straightforward, the key challenge lies in implementing the previously described assembly on the scale of a package-like stand-alone component. In other words, the entire force delivery system and the alignment scheme must be incorporated in the package substrate assembly itself, yet the package must be small enough (in size and weight) so as to be suitable for a MCM application. Furthermore, the package must also provide a means of environmentally protecting the die. Finally, the design of the assembly must ensure the long term reliability of the pressure connections.
A reliable, low cost electrical assembly which provides excellent electrical contact wherein assembly components can be readily demounted and reworked without damaging the die or substrate is needed.